In some networks based on 3GPP (Third Generation Partnership Project) (e.g., WCDMA (Wideband Code Division Multiple Access), a communication device (CD) receives and transmits data on only one carrier frequency (or “carrier” for short). Due to non-orthogonality between users, which results in interference leakage between the users, the uplink throughput is limited to 2-3 Mbps in scenarios with multiple users.
More in detail, in a WCDMA system as well as in a CDMA system, all users share the same uplink radio resources and can access the system at the same time. Each user is protected from others to a certain extent by using different scrambling codes. The protection is, however, not perfect. As mentioned, significant interference still leaks from one user to another due to the non-orthogonality between the users. New improvements in WCDMA, viz, high-speed packet access (HSPA) and HSPA+, have enabled very high single-user bitrates. The very-high bitrates introduce very high interference that makes it difficult for other users to co-exist in the same cell. For example, those that are farther away in the cell may not have sufficient uplink link budgets to overcome the high interference. This means, as concluded above, that in reality the high bitrates can rarely be used in real network and mixed traffic environments.
To enable high-bitrate operation in a real-network environment, the high-bitrate transmissions must be isolated from users that are vulnerable to the high interference created. A natural way to accomplish this within the current WCDMA technology is to make use of a “clean carrier” concept. In this concept, carriers are divided into regular carriers and clean carriers. The regular carriers provide the basic needs of a user. The clean carriers are dedicated exclusively to high-bitrate transmissions. On a clean carrier, users are scheduled by the network to transmit one at a time as much as possible in order to avoid interfering with one another.
One method of implementing a clean carrier system is to make use of the Inter-Frequency Handover (IFHO) procedure. Users are admitted on the regular carriers where user bitrates are limited to a certain maximum value. When there is a need for higher bitrates, the CD is reconfigured to a dedicated high-bitrate carrier using the IFHO procedure. When the need for high bitrates disappears, the CD is reconfigured back to a regular carrier.
Another method (see e.g. WO 2009/157836) is to make use of the 3GPP Rel-9 Multi-Carrier High Speed Uplink Packet Access (HSUPA) feature (also known as Dual Cell Enhanced Dedicated CHannel, E-DCH operation). A CD equipped with this capability can transmit simultaneously on two carriers: a primary carrier and a secondary carrier. The primary carrier would be configured on a regular carrier and the secondary on a dedicated high-bitrate carrier. When there is no high-bitrate need, the
CD transmits data only on the primary carrier. When there is the need, the CD can be scheduled to transmit on the secondary carrier in addition to the primary carrier.
The methods described above have a number of drawbacks in terms of performance and network management complexity. For example, the IFHO method has a disadvantage in that it involves re-allocating system-internal resources in addition to radio resources and is rather expensive in terms of network processing load. Further, the IFHO procedure requires Radio Resource Control (RRC) signaling, which introduces significant delay on the order of hundreds of milliseconds. If the user requires only a few Transmission Time Intervals (TTIs) of high-bitrate transmissions, it seems hardly worth the effort to perform the procedure. In addition, the delays introduce large overheads if each dedicated high-bitrate carrier is to be used by one user at a time. Still further, to start low-rate services, e.g. speech, while a CD is transmitting on a dedicated high-bitrate carrier may require that the connection be reconfigured back to a regular carrier.
The multi-carrier HSUPA method provides improvements, but still has a disadvantage in the delay in activating and deactivating the secondary carrier: 3GPP mandates a 6 TTI (18 slot) delay after receiving the High-Speed Shared Control CHannel (HS-SCCH) order. This delay is very significant when users are to be scheduled one at a time for short durations of a few TTIs. For good performance, it is preferable to have the secondary carrier activated at all times. Overhead for activating the secondary carrier for standby purpose is a further disadvantage. For inactive users, there is a cost in terms of higher CD battery consumption. The UL (UpLink) Dedicated Physical Control Channel (DPCCH) is transmitted on the secondary carrier for maintaining synchronization and power control. The CD also needs to monitor the Down-Link (DL) control channels on the secondary carrier. For the network, the interference from the many DPCCH from inactive users consumes considerable capacity of the secondary carrier. Furthermore, the interference on the secondary carrier is quite severe since it is intended for high-bitrate operations. The DPCCH power must be sufficiently high in order to maintain a good enough signal-to-noise ratio. When it is a user's turn to transmit, the interference from other users disappears and the signal-to-noise ratio of the DPCCH can easily jump 10 fold. This means the data part is also transmitted at 10 times the necessary power since the DPCCH power is used as a reference for setting the power of the data channel. This increases the CD's power consumption and decreases the maximum bitrate, as the CD can easily become power limited. A still further disadvantage is maintaining mobility on the secondary carrier: The CD is required to perform intra-frequency measurement and reports intra-frequency events to the network, even if the secondary carrier is deactivated.